Prescriptive vs. Performance Building Energy Codes

Most building energy codes today offer two paths for compliance: Prescriptive or Performance.  In this post, we’ll discuss the two different paths and why many projects follow the Prescriptive path even though the Performance path is probably more cost-effective.

Typical Code Structure

A prescriptive code requires that each component is built to a certain standard, e.g. Wall R-value at least 20.  A performance code requires that the building as a whole performs to a certain standard, e.g. uses less energy than the same building built to prescriptive code.

The International Energy Conservation Code (IECC), which is the basis for most building energy codes in the US, has had both Prescriptive and Performance paths since its inception in 2000.  IECC codes, which are typically updated every 3 years, have some mandatory provisions that all buildings must meet (e.g. proper mechanical equipment sizing).  Provided the mandatory requirements are met, the rest of the code can be met in one of two ways.  The project team can follow prescriptive requirements, building each component to a certain level of performance (some of the 2012 IECC prescriptive requirements are shown below).  Or they can prove that the building will perform at least as well as if it followed the prescriptive requirements, satisfying the performance path.  The performance path of the 2012 IECC requires that the proposed design be shown to have an annual energy cost that is less than or equal to the standard (prescriptive) design.

Prescriptive vs. Performance

Statistics are not readily available on the proportion of projects which follow the Prescriptive or Performance paths, but here at Ekotrope, it has been our experience that the majority go Prescriptive.  However, in developing and using our software, we have found that there are often thousands or tens of thousands of dollars in construction and material costs to be saved by following the performance path.  The reason people don’t follow the Performance path is that it requires either an energy rater or complex energy modelling software (or both).  The Ekotrope product provides a simple mechanism for following the Performance path and is recognized by RESNET as an Accredited IECC Performance Verification Software Tool.

Summary

There are two different paths to satisfy energy codes.  The Prescriptive path is simpler and can be achieved without the aid of any software.  The Performance path is more complicated but can result in substantial savings.  Ekotrope software supports the Performance path.